For a considerable period of time efforts have been directed at recovering the metal nitrites such as sodium nitrite, potassium nitrite, lithium nitrite, or nitrite mixtures containing at least one of the foregoing, from the reaction mixture effluent stream resulting from the manufacture of synthetic resins. One such manufacturing operation, for example, is that in the synthesis of aromatic bis(ether phthalimide) for the production of polyetherimide (PEI) plastics. Of particular interest is the product of a concentrated sodium nitrite solution that is the reaction by-product from the production of polyetherimide resin that contains the order of about one percent total organics; consisting of several organic compounds, including residual organic raw materials, organic catalysts and organic by-products.
Various techniques have been used to recover a constituent from a reaction mixture, one of which includes a solid-liquid separation technique of filtering at a temperature at which the constituent is substantially completely soluble while the alkali metal salt impurities are substantially insoluble. As described, for example, in U.S. Pat. No. 5,068,353, attention has generally been paid primarily to the recovery of the resin products, while the nitrite byproducts have been intermixed with raw materials, catalysts and organic by products mixed in a caustic wash and discarded as waste. More recent attempts to recover a nitrite by-product, such as those represented by U.S. Pat. No. 6,251,354B1, which describes methods for improving the quality of the nitrite stream typically generated with a “caustic wash” by utilizing a technique referred to as “water wash”, have provided limited improvement over the prior art in effecting a substantially complete recovery of the nitrite content, free of contaminants. Without further treatment, the residual organics in the water wash containing the nitrite impart undesirable properties, including a deep, dark color and a conspicuous odor to the solution. Another objectionable trait is the water wash solution's tendency to produce foam when agitated, which contributes disagreeable handling qualities. Consequently, complete removal of the organic impurities and these associated undesirable qualities is essential to providing for a commercially acceptable sodium nitrite solution. While techniques to provide for more concentrated and less-contaminated nitrite streams have been identified, the residual organic contaminants heretofore retained in the effluent stream have been persistent and have resisted the attempts to completely separate organics from the aqueous nitrite stream. While various techniques are proposed in U.S. Pat. No. 6,251,354B1 for removing the last traces of residual organic matter, such appear to be only partially successful, resulting in nitrite streams which contain the persistent residual organic compounds or other impurities, introduced by or during the processing, that render nitrite streams inappropriate for commercial use. For example, U.S. Pat. No. 6,251,354B1, teaches the use of activated carbon for removal of the residual organic material from both the water wash aqueous phase sodium nitrite stream, and the permeate from treatment of said stream representing pre-treatment with membrane separation. As represented by TOC measurements, activated carbon removed only 28.6% of the organic matter in the water wash and 65.5% of the residual organic matter in the lower-organic-containing membrane permeate. The higher removal efficiency demonstrated by this unidentified carbon on the permeate versus the water wash suggests that lower starting levels and/or smaller organic molecules (that tend to pass through the membranes while the larger organic molecules are separated), gave improved performance. Still, this incomplete removal does not provide for a purified sodium nitrite stream suitable for commercial use. Also proposed among the purification techniques of that patent is a melt process that results in the destruction of nearly all of the organic matter as the solution is dried and then heated to very high temperatures. While this technique leads to low residual organic matter in the resultant nitrite product, a significant amount of carbonate impurity is introduced during the process, thus in effect trading one impurity for another. Because the existing technology for recovery of the nitrite byproducts has not provided for a usable or salable quality of said byproduct, there is a definite need in the art for a method that enables the recovery of useable metal nitrite content, in particular, sodium nitrite from the reaction mixture formed from the synthesis of aromatic polyetherimide resins.